Re: optimisation question

[Max Ischenko <max@...>]

> > I have a function I would like to optimize (yes, I've done profiling).
> > it transforms string like "3-7" into "3,4,5,6,7".

> Try this:

> function transform(str)
>   local _, _, from, to = string.find(str, "^(%d+)%-(%d+)$")
>   assert(from, "Transform needs a string in the format i-j")
>   local n1, n2 = tonumber(from), tonumber(to)
>   if n1 >= n2 then return from end
>   local template = from .. string.rep(", ", n2 - n1)
>   return (string.gsub(template, " ", function() n1 = n1 + 1; return n1 
> end))
> end

Hmm, interesting trick -- using closure in repl fn!
I 'd never thought about this.

> Philippe had a good idea about memoising results; the above can be
> improved (slightly) by memoising the template string. (It only helps
> on largish ranges.)

> -- A useful tool:

> function memoise(fn)
>   return setmetatable({}, {
>     __index = function(t, k)
>                 local rv = fn(k)
>                 t[k] = rv
>                 return rv
>               end
>   })
> end

[ ... mor cool stuff skipped ... ]

> The test for n1 >= n2 is actually unnecessary, but that counts on a
> particular behaviour of string.rep. In fact, the conversion of from
> and to to numbers is also unnecessary (if the comparison is removed),
> because it will happen automatically. So making those changes, we
> could reduce the code to the following: (shorter, if not faster)

At the end, I must say that I've learned a lot about Lua idioms and tips
while solving this optimisation problem.

Many thanks to you and others for their participation and
very enlightening comments!

-- 
Regards, max.